1. Field of the Invention
The present invention relates to a polarized neutron guide, and more particularly, to a polarized neutron guide improved in its yield, solving a problem of a low yield of 50% in the case of a general polarized neutron guide.
2. Description of the Related Art
A neutron guide is a hollow tube consisting of glass plates deposited with Nickel or periodic multi-layer (super mirror) for transferring cold neutrons (referred to as neutrons hereinafter) generated from a cold neutron source in a vacuum state to a device located for a long distance with a minimum loss. Referring to FIG. 1, the neutron guide 200 is formed at a desired length extended to a device by serially connecting a plurality of guide units 210.
Referring to FIG. 2, the guide units 210 of the neutron guide 200 have a length of about 1 m, with the super mirrors being assembled in a long-box shape having a quadrangular cross-section.
In the super mirrors 212, which are capable of enlarging the total reflection angle by two folds or more, a magnetic material of high scattering length density (Ni, Fe, Co) and a non-magnetic material of low scattering length density (Si, Ti, Cu) are selectively deposited on each substrate 212a to form a thin film 212b as a reflective plane on a surface facing the inner path formed by the guide units 210.
Therefore, the guide units 210 allow neutrons to be total-reflected within a critical angle in the inside by the super mirrors 212 formed by thin-film deposition.
The neutrons in most of elements except some elements (e.g., Gd, Mn) have a positive (+) scattering-length density, which means that an incident angle of neutrons between an incident direction and a medium surface is greater than a refractive angle in the medium unlike electromagnetic waves in a visible light region. Such special property of neutron and an X-ray means that the neutron and the X-ray can be total-reflected from a medium when they are incident on the surface of the ideal material (medium) within a critical angle.
Therefore, a basic concept of 58Ni neutron guide capable of moving, i.e., transferring neutrons using the total-reflection property of the neutrons has been suggested in the related art. Since then, a super mirror guide has been used as a neutron guide formed with using the natural nickel and titanium (58Ni: 68%).
Neutrons, electrons, X-rays tend to be diffracted in a structure of periodically repeated crystal planes of a crystal of an atom or a molecule. Diffraction can be observed in a thin-film structure where two different materials are artificially repeated periodically.
A theory that a diffracted line width can be widened up to a critical angle by changing the thickness of repeated multi-layered thin films has been introduced. A medium capable of widening a total reflection angle of nickel more than two times by applying the above theory is a super mirror 212, which is used for a neutron guide 200.
To transfer the neutrons generated from a cold neutron source 300 up to a remotely located device 310 without loss of the neutrons, a neutron guide 200 in a vacuum state is used. As described above, the related art neutron guide 200 uses the property that neutrons are total-reflected when they are incident on the surface of a material (medium) within a critical angle.
Neutrons transferred through the neutron guide 200 may sometimes require the spin of the neutron biased in one direction. Using ferromagnetic material and non-magnetic material for the super mirror to form the multi-layer thin film, spin-up polarized neutrons can be separated from spin-down polarized neutrons. In this case, only the type of spin needed for the corresponding apparatus 310 should be used and the rest of the types should be separated to be discarded.
To polarize and supply neutrons, a polarized neutron guide is required. The polarized neutron guide can be made of alloys of ferromagnetic materials. A neutron due to its own magnetic moment, has any of two spin directions i.e. a spin-up direction parallel with the direction of a magnetic field and a spin-down direction unparallel with the direction of a magnetic field. The two spin directions of a neutron result in different scattering abilities for a magnetized material. It is possible to polarize a neutron using this property.
For the super mirror 212 of a neutron guide 200 for transferring neutrons, if the thin film 400 is composed of FeCo of the magnetic material 410 and Si of the non-magnetic material 412 at a ratio of 89:11, the down-spin neutron ultimately has the same scattering length density as that of Si, the non-magnetic material 412. Thus, when the thin film 400 is formed, the up-spin neutron 422 is diffracted or reflected whereas the down-spin neutron 422b cannot be diffracted but permeates due to the same scattering length density as that of Si, unable to distinguish between FeCo and Si, as illustrated in FIG. 3.
The polarized neutron guide may be a residual magnetic polarized guide. The residual magnetic polarized guide is formed so that a thin film 400 magnetized under a magnetic field does not lose magnetization thereof even though the magnetic field disappears afterward. The residual magnetic polarized guide is manufactured using a principle of a recording tape.
In the residual magnetic polarized guide, in order to easily perform magnetization, a thin film of FeCoV/TiZr is formed by adding foreign substance to FeCo alloy, or a thin film of FeCo/Ge is formed by using Ge instead of Si.
In order to divide and selectively supply neutrons 422 transferred by the neutron guide 200 into spin-up polarized neutrons 422a or spin-down polarized neutrons 422b, a conventional polarized neutron guide 500 was suggested as illustrated in FIG. 4.
The conventional polarized neutron guide 500 for generating polarized neutrons is connected at the front with the neutron guide 200 to receive neutrons 422, and separates the neutrons 422 into spin-up polarized neutrons 422a and spin-down polarized neutrons 422b. However, the conventional polarized neutron guide 500 has a disadvantage of collecting only selected polarized neutrons (e.g., spin-up polarized neutrons 422a, which is 50% of the neutrons 422 only), and losing non-selected polarized neutrons (e.g., spin-down polarized neutrons 422b, which is 50% of the neutrons 422), during this process.
Various materials can be used for manufacturing the polarized neutron guide 500 for polarizing and separating the neutrons. For representative example, an alloy of ferromagnetic material such as Fe and Co can be used with Si. A thin film of magnetic material 410 of for example FeCo alloy deposited on the surface of a super mirror 510 in the conventional polarized neutron guide 500 is magnetized inside a magnetic field of a magnetic field generating member 520 installed outside the polarized neutron guide 500. Neutrons 422 flowing into the polarized neutron guide 500 under the magnetic field are divided into spin-up polarized neutrons 422a and spin-down polarized neutrons 422b having different scattering length densities, respectively.
That is, since the scattering length density of the spin-down polarized neutrons 422b due to the magnetic material 410 of FeCo is matched with the scattering length density of the spin-down polarized neutrons 422b of Si, which is a non-magnetic material 412, regardless of a difference between the two materials 410 and 412, the spin-down polarized neutrons 422b are all transmitted below a critical angle of the super mirror 510 constituting the polarized neutron guide 500. On the contrary, the spin-up polarized neutrons 422a are diffracted and total-reflected by the super mirror 510 constituting the polarized neutron guide 500, and transferred inside the guide 500. With such a principle, the conventional polarized neutron guide 500 can polarize the spin-up polarized neutrons 422a only from the neutrons 422 to collect the same.
However, since the conventional polarized neutron guide 500 separates one kind of polarized neutrons, i.e., the spin-up polarized neutrons 422a only without collecting the spin-down polarized neutrons 422b, the neutrons 422 are used in 50% only in viewpoint of the whole collecting efficiency.
Unlike the polarized neutron guide 500 consisting of super mirrors 510 using the above-described related art magnetic material 410 and non-magnetic material 412, i.e., FeCo/Si, a neutron inverse-polarization guide (not shown) using super mirrors of a Co/Cu has been suggested, which is designed to transmit and remove the spin-up polarized neutrons 422a, while reflecting and collecting the spin-down polarized neutrons 422b. 
Therefore, conventionally, where the spin-up polarized neutrons 422a or the spin-down polarized neutrons 422b is required respectively, the polarized neutron guide 500 made of FeCo/Si for separately collecting the spin-up polarized neutrons 422a is used, or a spin-flipper for separately obtaining the spin-down polarized neutrons 422b is used. These polarized neutron guides are very expensive, and require a precise treatment but considered inefficient, since the yield of neutron is only about 50%.